1. Field of the Invention
The present invention is directed to a new architecture for an all-optical fiber or waveguide switch based on a fiber Sagnac interferometer.
2. Description of the Related Art
In an all-optical waveguide switch, a light signal is switched from one output port to another by the application of either another optical signal of different wavelength (pump-induced switching) or by the light signal itself (self-switching). This is typically accomplished in an optical interferometer by placing an element possessing an optical third-order nonlinearity in one of the two arms of the interferometer. For example, in the case of pump-induced switching, in the absence of pump light, the interferometer is adjusted (or fabricated) such that all the signal power comes out of one of the two output ports of the interferometer. When the pump light is applied, it modifies the index of refraction of the nonlinear element, and thus the phase of the signal traveling in this arm. When the phase shift has the right value (which depends on the interferometer, but which is, as an example, xcfx80 in a Mach-Zehnder interferometer), the signal is switched from one port to the other.
Because third-order nonlinear effects are generally weak, they tend to require relatively high intensities and/or long nonlinear media to produce this kind of large phase shift. The switching is then characterized by a high intensity-length product. Thus, an optical fiber which preserves a high optical intensity over very long lengths (kilometers) can produce a large phase shift at low optical powers. In fibers, however, only a few types of third-order nonlinearities are available. The most commonly used type is the Kerr effect. The Kerr effect is, however, notoriously weak in silica fibers. To make a Kerr-based switch in a silica fiber requires either a long fiber and a relatively low switching power, or a high power and a short fiber (or waveguide). In the former situation, the fiber arm needs to be so long that most interferometers are unstable and impractical. This is particularly true of the commonly-used Mach-Zehnder interferometer, which needs to be in the sub-centimeter length range for its bias point to be stable over reasonable fiber temperature changes. In the latter situation, the fiber can be short and thus the interferometer can be more stable, but the power required to switch is too high. A high switching power is detrimental because it leads to breakdown of the fiber, because it is expensive, or both.
Other materials and other types of nonlinearity are much stronger than the Kerr effect in silica, and thus require smaller intensity-length products. One particular example is so-called resonantly enhanced nonlinearities, which occur in materials and/or dopants that possess suitable electronic transitions. Examples include semiconductors, such as CdSexSlxe2x88x92x, or GaAs, and chalcogenide glasses. (See, M. Asobe, Low power all-optical switching in a nonlinear optical loop mirror using chalcogenide glass fibre, ELECTRONICS LETTERS, Jul. 18, 1996, Vol. 32, No. 15, pp. 1396-1397.) A resonantly enhanced nonlinearity can also be observed in dopants that can be introduced into a silica fiber, for example, a trivalent rare earth like erbium (Er3+) or neodymium (Nd3+). (See, M. J. F. Digonnet, et al., Resonantly Enhanced Nonlinearity in Doped Fibers for Low-Power All-Optical Switching: A Review, OPTICAL FIBER TECHNOLOGY Vol. 3, 1997, pp. 44-64.) The advantage of the latter type of nonlinearity is that one can still utilize a silica-based fiber, i.e., retain all the basic low-loss, low-dispersion properties of the silica fiber, which may be eventually beneficial to produce a low-loss, ultrafast switch. However, with existing resonantly enhanced nonlinear materials, if one wishes to keep the switching power low, the length required for the nonlinear element is still too long for most interferometers to be stable.
In summary, the search for a suitable all-optical switch is strongly connected to (1) the development of materials with strong third-order nonlinearities, and to (2) the identification of a switch architecture that can be stable even with long lengths of fiber in its arms.
The Sagnac fiber loop was recognized years ago as a potential solution to this last problem. The primary reason is that unlike most interferometers, the Sagnac loop is a true commonpath interferometer, which means that it is reciprocal. Therefore, even with very long loop lengths, the Sagnac loop is extremely stable to slow external perturbations (slow being defined on the scale of the time it takes light to propagate around the Sagnac loop). Thus, it is possible to utilize a very long Sagnac loop of silica fiber (up to kilometers) and obtain, via the Kerr effect of the fiber, a sizeable phase shift with a low switching power.
The Sagnac interferometer has been used in several ways to demonstrate all-optical switching. The most common approach utilizes the Kerr effect of the silica fiber and an effect known as cross-phase modulation. (See, N. J. Doran, et al., Experimental Investigation of All-Optical Switching in Fibre Loop Mirror Device, ELECTRONICS LETTERS, Vol. 25, No. 4, Feb. 18, 1989, pp. 267-269; and M. C. Farries, et al., Optical fiber switch employing a Sagnac interferometer, APPLIED PHYSICS LETTERS, Vol. 55, No. 1, Jul. 3, 1989, pp. 25-26.) In this scheme, the pump pulse that causes the switching propagates only in one direction of the loop, and the pump pulse is much shorter than the loop length. The signal traveling in the loop in the same direction as the pump (copropagating) sees the pump during its entire passage through the loop, while the signal traveling in the other direction as the pump (counterpropagating) sees the pump only during the brief time they happen to be at the same location in the loop. Since the Kerr effect is extremely fast (femtoseconds), for pump pulses 100 femtoseconds or longer (which covers most experimental situations), the counterpropagating signal experiences a nonlinear index change over a very short fraction of the loop length. On the other hand, the copropagating signal experiences a nonlinear index change over the entire loop length (assuming negligible walk-off). Thus, the two signals experience a differential phase shift. When the pump power is such that this differential phase shift is equal to xcfx80, the signal has been fully switched from one port to the other.
A self-switching application of the Kerr effect in a Sagnac loop utilizes the fact that if the two signals counterpropagating in the loop have different powers, which can be induced by adjusting the coupling ratio of the Sagnac loop coupler away from 50%, then one signal will experience a larger Kerr phase shift than the other. (See, N. J. Doran, et al., cited above.) By adjusting the signal power, this power imbalance can be such that the differential phase shift between the counterpropagating signals is xcfx80, and again the signal is fully switched.
Another embodiment utilizes the Kerr effect again but counterpropagating signals with orthogonal polarizations in the Sagnac loop. (See, M. Jinno, et al., Demonstration of laser-diode-pumped ultrafast all-optical switching in a nonlinear Sagnac interferometer, ELECTRONICS LETTERS, Vol. 27, No. 1, Jan. 3, 1991, pp. 75-76.) The loop is made of polarization-maintaining fiber to ensure. that the polarizations of the two optical signals and the pump remain the same relative to each other along the entire loop. The signal with a polarization parallel to the pump polarization then experiences a larger phase shift than the signal with a polarization orthogonal to the pump polarization. Again, by adjusting the pump power to. a suitable level, this differential phase shift can be made equal to xcfx80, and the signal is fully switched. This effect was also demonstrated using a dye-doped polymer fiber as the nonlinear element. (See, D. W. Garvey, et al., Characterization of the Switching Properties of a Singlemode Polymer Optical Fiber, SPIE, Vol. 2527, 1995, pp. 404-410.)
Another demonstration uses a chalcogenide fiber as the nonlinear element, which is inserted in a Sagnac loop made of a silica fiber. (See, M. Asobe, et al., cited above.) The use of the chalcogenide fiber, which has a much stronger Kerr effect than silica, enables the use of a shorter fiber and/or a lower switching power.
In another embodiment, a fiber Sagnac switch was demonstrated in which the nonlinear element was a D-shaped fiber coated with xcex1-silicon, a semiconductor that acts as a nonlinear material. (See, R. M. Ribeiro, et al., Switching in all-fibre interferometer using a semiconductor coated D-fibre, ELECTRONICS LETTERS, Vol. 32, No. 15, Jul. 18, 1996, pp. 1402-1403.) The D-shaped fiber was placed asymmetrically in the Sagnac loop, close to the coupler. Because of this asymmetry, the signal that arrives at the nonlinear element first experiences a certain phase shift. If the nonlinear response of the nonlinear element is much shorter than the loop transit time, and if the pump is turned off by the time the counterpropagating signal arrives at the nonlinear element, then the counterpropagating signal, which arrives later, will experience a nonlinear phase shift that is lower (ideally zero) than the phase shift experienced by the first signal.
All of the Sagnac loop switches reported to date, however, still utilize relatively long lengths of fiberxe2x80x94generally tens of centimeters or more. They also require very fast nonlinear media.
It is the purpose of this invention to provide a Sagnac interferometer which can be used with relatively slow nonlinear media, as well as with media in which the pump-induced index change occurs via a thermal effect. The present invention is particularly attractive to produce switches that need to remain xe2x80x9conxe2x80x9d for relatively long times (from -nanoseconds to microseconds). Unlike other Sagnac switches, the xe2x80x9conxe2x80x9d time can be conveniently adjusted by changing the length of the Sagnac loop.
The Sagnac switch architecture in accordance with the present invention is stable against slow environmental perturbations such as temperature changes for any length of waveguide, even for very long waveguides. This property of the present invention makes it possible to use longer waveguides of any length, provided the active (e.g., doped) portion of the waveguide changes its index of refraction very rapidly in response to the initiation of pumping and the active portion of the waveguide returns to its original index of refraction very slowly after the pumping ceases. The present invention uses the delay in a Sagnac loop to cause the switching off and to control the on time of the switch.
One aspect of the present invention is an apparatus for providing all-optical switching of an optical signal. The apparatus includes an input waveguide which receives an input optical signal. A loop of optical waveguide has an active portion located asymmetrically in the loop. A coupler couples light from the input waveguide to the loop to cause the optical signal to propagate in the loop as first and second counterpropagating signals. The coupler also couples the first and second counterpropagating signals from the loop as a combined output signal. The coupler has first and second output ports. The coupler couples the combined output signal to the first output port when the first and second counterpropagating signals coupled from the loop have a first phase relationship. The coupler couples the combined output signal to the second output port when the first and second counterpropagating signals coupled from the loop have a second phase relationship. A source of pump light is coupled to the loop to introduce pump light to the active portion of the loop. The active portion of the loop is located asymmetrically in the loop. The active portion of the loop is made of a material which has an index of refraction that is intensity dependent. When pump light of suitable wavelength is launched into the active portion, the pump light causes an index change which causes phase changes in the first and second counterpropagating signals. The phase changes cause the first and second signals coupled from the loop to switch from the first phase relationship to the second phase relationship for a time duration proportional to a propagation time through the loop, after which the first and second signals coupled from the loop return to the first phase relationship.
Another aspect of the present invention is a method for switching an optical signal using an optical pump. An optical signal is input into a loop as first and second counterpropagating signals. An active portion of the loop is pumped with the optical pump. The active portion is located asymmetrically in the loop. The pump causes the active portion of the loop to modify the phases of the first and second counterpropagating signals. The location of the active portion in the loop causes the first counterpropagating signal to exit the loop with a modified phase before the second counterpropagating signal exits the loop with the modified phase. The method further includes the step of interfering the first counterpropagating signal with the second counterpropagating signal at a coupler having first and second output ports to generate an output signal. The output signal is output from the second port of the coupler when only one of the counterpropagating signals at the coupler has the modified phase. The output signal is output from the first port of the coupler when neither of the counterpropagating signals at the coupler has the modified phase or when both of the counterpropagating signals at the coupler have the modified phase.
Another aspect of the present invention is a method of using a Sagnac interferometric loop as an optical switch. An input optical signal is provided to a first port of the interferometric loop to cause two portions of the input optical signal to counterpropagate in the interferometric loop. A pump signal is selectively coupled to an asymmetrically located active portion of the loop. The pump signal causes the active portion of the loop to change propagation characteristics. Signal light is output from the interferometric loop. The signal light results from combining the two portions of the input optical signal counterpropagating in the interferometric loop. The signal light is output from the first port before the pump signal is coupled to the active portion of the interferometric loop, The signal light is output from a second port of the interferometric loop when only one of the two portions of the input optical signal has passed through the active portion of the interferometric loop. The signal light is again output from the first port of the interferometric loop when both portions of the input optical signal have passed through the active portion of the interferometric loop.